p-i-n Photodiodes

ABSTRACT

Photodiodes (10) are fabricated in a single step diffusion process which exploits the characteristic of certain acceptors to form an anomalous diffusion profile (VI) including shallow and deep fronts (VIa and b) joined by an upwardly concave segment (VIc). By performing this type of diffusion into a low-doped n -  -type body (12) with a carrier concentration (VII) below that of the concave segment, a p +  -p -  junction (15) is formed at the depth of the concave segment and a p -  -n -  junction (17) is formed at a greater depth. The zone (16) between the junctions is at least partially depleted and forms the active region of a p +  -p -  -n -  photodiode. Specifically described are InP:Cd photodiodes.

This is a continuation, of application Ser. No. 154,046, filed May 28,1980.

BACKGROUND OF THE INVENTION

This invention relates to detectors of optical radiation (i.e.,lightwaves) and, more particularly, to semiconductor photodiodes.

The recent special issue of the Western Electric Engineer, Vol. XXIV,No. 1, Winter 1980, is a graphic illustration of the burgeoning interestin lightwave communications systems, especially fiber optic systems. Therapid growth of these systems has engendered commensurate activity inoptical sources and detectors, primarily GaAs-AlGaAs laser diodes andLEDs in conjunction with Si APDs and p-i-n diodes for presentapplications at relatively short wavelengths (e.g., 0.80-0.90 μm), andInP-InGaAsP laser diodes and photodiodes for future systems at longerwavelengths (e.g., 1.1-1.6 μm).

In general, a photodiode operates by the absorption of light whichgenerates electron-hole pairs in the depletion region of a p-n junction.In the photovoltaic mode or under reverse bias, the junction fieldseparates the pairs and thereby produces a photocurrent which can bemade to do useful work in an external load. The optical-to-electricalconversion efficiency can be enhanced by employing a p-i-n photodiodeconfiguration in which the impurity concentration of the i-layer is lowenough to produce complete depletion. A depleted i-layer, often calledthe active layer where light is primarily absorbed, means that pairs canbe readily separated and do not recombine before producing a usefulphotocurrent. Although the i-layer should be made thick enough to absorba substantial fraction of the light incident thereon, it is often madeeven thicker to reduce leakage current, increase the reverse breakdownvoltage, and lower the capacitance of the photodiode. On the other hand,the maximum thickness of the i-layer is limited primarily by therequired speed of operation.

Realizing low-doped i-layers can often be a problem depending on thematerials from which the photodiode is made and the fabricationtechniques employed. For example, suitable p-i-n photodiodes can be madeof silicon using, inter alia, high resistivity (>300 Ω-cm) epitaxiallayers and ion-implantation (see, U.S. Pat. No. 4,127,932 granted to A.R. Hartman et al), yet the ability to controllably fabricate similardevices from Group III-V compound semiconductors is complicated by thedifficulty of making low-doped material (e.g., 10¹⁵ cm⁻³). This problemin turn limits the maximum depletion width attainable and hence placesconstraints on desired levels of leakage current, breakdown voltage, andcapacitance. There is a need, therefore, to be able to fabricaterelatively wide (e.g., 10 μm) depletion layers in more highly doped(e.g., mid-10¹⁶ cm⁻³) Group III-V compound semiconductors.

The fabrication of prior art photodiodes is also disadvantageous becauseof the need to grow a plurality of epitaxial layers of controlledcomposition, conductivity type and thickness often involvingsophisticated growth procedures (e.g., LPE, MBE, VPE) and a complicatedsequence of ion-implantation and/or diffusion steps. So, there is also aneed to simplify the fabrication of photodiodes and, thereby, toincrease reproducibility and reduce costs.

SUMMARY OF THE INVENTION

We have devised a relatively simple, reproducible and inexpensiveprocedure for fabricating a p-i-n photodiode in moderately lowly dopedGroup III-V compound semiconductor n⁻ -type body in which a singleacceptor diffusion step produces both the p-i junction and the i-njunction separated by a fully depleted, relatively thick p⁻ activei-region. Depleted active regions thicker than 10 μm can be controllablyfabricated in semiconductor bodies having a carrier concentration ashigh as the mid-10¹⁶ cm⁻³ range.

More specifically, we exploit the characteristic of certain acceptorsunder particular conditions to deviate from expected error functiondiffusion profiles and to form an anomalous concave section resulting intwo diffusion fronts. This anomalous profile includes upper and lowermonotonically decreasing segments joined by the upwardly concavesection, with the upper segment being closer to the surface and thelower segment being deeper. By performing this type of anomalousdiffusion into an n-type body doped to a carrier concentration belowthat corresponding approximately to the concave section, a p⁺ -p⁻junction is formed at a depth corresponding approximately to that of theconcave section, and a p⁻ -n⁻ junction is formed at a depthcorresponding to the intersection of the lower segment and the carrierconcentration of the n⁻ -body. The p⁻ -region between the junctions isdepleted and forms the active i-region where light is absorbed andphotocarriers are generated. The thickness of the active region iscontrolled by controlling the difference in doping level of the body andthat of the concave section.

Although numerous prior art workers have studied these anomalousdiffusion profiles, none has fabricated photodiodes incorporating thecharacteristic. Representative of the prior art studies are thefollowing:

    ______________________________________                                        GaAs:Zn    B.    Tuck et al, Journal of Materials                                              Science, Vol. 7, page 585 (1972)                             InP:Zn     B.    Tuck et al, J. Phys. D: Appl. Phys.,                                          Vol. 8, page 1806 (1975)                                     GaP:Zn     L.    L. Chang et al, J. Appl. Phys., Vol. 35,                                      page 374 (1964)                                              GaAs:Mn    M.    S. Seltzer, J. Phys. Chem. Solids,                                            Vol. 26, page 243 (1965)                                     Ge:Cu      F.    Van der Maesen et al, J.                                     Ge:Ni            Electrochem. Soc., Vol. 102,                                                  page 229 (1955)                                              CdS:Ag     H.    H. Woodbury, J. Appl. Phys.,                                                  Vol. 36, page 2287 (1965)                                    InP:Cd     B.    I. Miller and P. K. Tien, 21st EMC,                                           Abs. No. G4 (6/1979)                                         InP:Cd     P.    K. Tien and B. I. Miller, Appl. Phys.                                         Lett., Vol. 34, page 701 (5/1979)                            ______________________________________                                    

Indicative of the failure of prior art workers to recognize the utilityof the anomalous diffusion profile in, for example, InP:Cd or Zn, is thereport by T. P. Lee et al, Appl. Phys. Lett., Vol. 35, page 511(10/1979). This paper describes InP photodiodes in which Zn or Cddiffusion was employed to make simple p-n junctions but, notwithstandingthe earlier work of Tuck (1975) and the two works of Miller and Tien(1979), did not exploit the anomalous diffusion of these acceptors inany way.

In contrast, we fabricated abrupt p⁺ -p⁻ -n⁻ junctions in nominallyundoped InP n⁻ -substrates under conditions that produced the anomalousdouble diffusion profile. The shallow front (i.e., the p⁺ -p⁻ junction)marked the concave section of the profile, and the deep frontcorresponded to the p⁻ -n⁻ junction. The depletion region extendedbetween the two fronts. To achieve this profile, we found the Cdactivity should be less than about 0.15 at a substrate temperature of680 degrees C. Unlike previous work which used Cd compounds (Tien andMiller, supra), we diffused Cd into n-type InP (4×10¹⁶ cm⁻³) in sealedampoules using elemental sources. The diffusion depths were controlledby varying the diffusion time or the concentration of either In or P.

Mesa photodiodes characterized by large reverse breakdown voltages, lowleakage current, the wide depletion widths were fabricated using thistechnique. These photodiodes are suitable for detection of light atwavelengths λ≦0.96 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

Our invention, together with its various features and advantages, can bereadily understood from the following more detailed description taken inconjunction with the accompanying drawing in which:

FIG. 1 is a cross-sectional view of a mesa photodiode in accordance withone embodiment of our invention;

FIG. 2 is a graph of diffusion depth in InP versus the In and Pconcentration of the diffusion source in which the Cd concentration,temperature, and ampoule volume were 3.5 mg, 680 degrees C., and 5 cm³,respectively; and

FIG. 3 is a calculated graph of the normal error function diffusionprofile (curve V) and the anomalous double diffusion profile (curve VI).For n⁻ =4×10¹⁶ cm⁻³ (line VII) substrate doping level, one junction (p⁺-n⁻) would be formed at a depth d₁ for a normal diffusion profile,whereas two junctions (p⁺ -p⁻ and p⁻ -n⁻) are formed at d₂ and d₃ for ananomalous profile.

DETAILED DESCRIPTION

With reference now to FIG. 1, there is shown a p-i-n photodiode, e.g.,p⁺ -p⁻ -n⁻ photodiode 10 which, in accordance with one aspect of ourinvention, is fabricated by means of a single diffusion step performedunder conditions which result in an anomalous double diffusion profile.The photodiode 10 includes a relatively low-doped n⁻ -type semiconductorbody 12, which may be a substrate of single crystal semiconductor orsuch a substrate with one or more epitaxial layers (not shown) grownthereon. By suitable single step acceptor diffusion describedhereinafter, a surface layer 14 of body 12 is converted to p⁺ -type andan underlying layer 16 is converted to p⁻ -type. The latter layer ofthickness t is the active region of the photodiode where light 18 isprimarily absorbed and generates photocarriers. These carriers arecollected by suitable electrical contacts, illustratively a broad areacontact 20 on the bottom of body 12 and an annular contact 22 on the topof layer 14. Light 18 passes through the annulus of contact 22 to theactive region. Typically, for high speed detection a reverse bias isapplied across contacts 20 and 22. The photocurrent, whether generatedin the photovoltaic or reverse bias mode, then can do work in anexternal load (not shown) connected across these contacts.Illustratively, photodiode 10 is formed in the shape of a mesa,typically to reduce device capacitance.

In order to realize an anomalous diffusion profile, such as shown bycurve VI of FIG. 3, three basic conditions should be observed: (1) thediffusing acceptor should have a high diffusivity, e.g., a fastdiffusing impurity such as Cu or Ni in Ge and Zn or Cd in Group III-Vcompounds; (2) the surface concentration of the impurity should berelatively high (e.g., in the upper 10¹⁸ cm⁻³ range), and (3) thediffusion time should be relatively long; i.e., more than a few minutesto insure that acceptors have time to penetrate deep enough into thebody to form the deeper p-n junction, but less than many hours toprevent equilibration; 1 to 4 hours is suitable for InP:Cd. Furthermore,in order to exploit the anomalous diffusion profile to make a p⁺ -p⁻ -n⁻photodiode, the carrier concentration (line VII, FIG. 3) of thelow-doped body 12 must be less than net impurity concentrationcorresponding to the concave section VIc. Under these circumstances, theshallow diffusion profile VIa produces a p⁺ -p⁻ junction at depth d₂corresponding approximately to the concave section VIc, whereas thedeeper diffusion profile VIb forms a p⁻ -n⁻ junction at depth d₃corresponding to its intersection with line VII. The width t=d₃ -d₂ ofthe active region between these junctions is controlled by thedifference in carrier concentrations between concave section (VIc) andbody 12 (line VII). Advantageously, this active region is at leastpartially depleted (because it is compensated) and can be maderelatively wide (e.g., 10 μm), thereby decreasing leakage current,increasing reverse breakdown voltage, and lowering capacitance.

EXAMPLE

The following example describes the fabrication of a p⁺ -p⁻ -n⁻ InPphotodiode by a single step diffusion of Cd.

In this example, Cd was diffused into n-type InP using elemental (Cd andP) or (Cd and In) as sources. p⁺ -p⁻ -n⁻ junctions were formed usingeither Cd source. Higher reverse breakdown voltages and lower reverseleakage currents, exceeding those of state-of-the-art, abrupt p⁺ -n⁻ InPavalanche photodiodes (APDs), were obtained from our p⁺ -p⁻ -n⁻ diodes.

Unintentionally doped (4×10¹⁶ cm⁻³) InP substrates (e.g., body 12) werecut along the <100> orientation from twin free, liquid encapsulated(LEC) grown crystals. One surface of the substrates was polished usingBr-methanol in order to accurately determine the diffusion depth and toqualitatively assess the degree of thermally induced decomposition ofthe surface. The diffusion anneals were carried out in sealed quartzampoules for four hours at 680 degrees C. in a vertical furnace with thediffusion source at 675 degrees C. to prevent condensation of dropletsonto the InP wafer. The ampoule volume and amount of Cd were heldconstant from run to run at 5 cm³ and 3.5 mg, respectively. Thediffusion depth was controlled by varying the diffusion time or theconcentration of either In or P. The diffusion front was revealed bystaining a cleaved edge with a well-known AB etch at room temperaturefor five minutes. Examination of the diffused samples using Nomarskiinterference microscopy showed no significant change in surface quality.Similarly, photoluminescence measurements of the diffused samples wereconsistent with the absence of surface damage.

A Nomarski optical micrograph was made to observe the typical stainededge of a nominally undoped, n-type InP (4×10¹⁶ cm⁻³) wafer after Cddiffusion. Two diffusion fronts 15 and 17 (FIG. 1) were formed, one at adepth d₂ of 11.4 μm and the other at a depth d₃ of 24.6 μm for an amountof P in the diffusion source equal to about 3.4 mg. The two fronts wereeither changes in doping density or dopant type since etching revealedboth types of variations. Examination of the stained edge with ascanning electron microscope (SEM) showed that a step, ˜0.2 μm deep, wasdelineated at the shallow diffusion front 15 for five minutes etch time;no step was observed at the deep front 17 even after thirty minutes etchtime.

A high magnification electron beam induced current (EBIC) image of thesample was also made, and the positions of the shallow and deepdiffusion fronts obtained from the optical micrograph were compared.This EBIC profile demonstrated the nature of the junction. The EBICsignal reached a peak at the shallow front 15, remained at the peakvalue for ˜3 μm, and decreased to a second inflection point at theposition of the deep front 17. The EBIC measurement thus revealed thepresence of a depletion region (active region 16) bounded by the twofronts. The EBIC profile was characteristic of a p⁺ -p⁻ -n⁻ junction.

Further investigation of the active region 16 bounded by the twodiffusion fronts was made using thermal probing and photoluminescence.Using a 2 percent Brmethanol solution, a surface layer, approximately 20μm thick, was removed from a top portion of the wafer. A thermal probemeasurement of the original surface of the sample showed it to bestrongly p-type. Thermal probing of the etched surface at 20 μm depthshowed it to be low doped; the conductivity type could not bedetermined. Low temperature (4.2 degrees K.) photoluminescencemeasurements also indicated a high Cd concentration at the originalsurface. Similar photoluminescence measurements of the etched surfaceindicated that it was p-type as shown by the width and strength of theCd impurity line at 1.365 eV. An amount of Cd, lower than the surfaceconcentration by about an order of magnitude was found. The formation ofa p⁺ -p⁻ -n⁻ junction was thus confirmed by EBIC, thermal probing, andphotoluminescence measurements.

FIG. 2 summarizes the results of diffusing Cd into undoped (4×10¹⁶ cm⁻³)n⁻ -type InP. The diffusion depths for both shallow and deep frontsversus the amount of either P or In in the source are plotted; i.e.,curves I and II correspond to the shallow p⁺ -p⁻ junction 15 and thedeeper p⁻ -n⁻ junction 17, respectively, for an In-Cd source, whereascurves III and IV represent the same junctions for a P-Cd source. Forexample, at 20 mg of In, the distance between curves I and II gives a p⁻-active region 16 which is about t₁ =8 μm thick. Similarly, at 20 mg ofP, the active region thickness is about t₂ =10 μm. The data show thatthe diffusion depths decrease for increasing amounts of either P or Inand that the dependence of diffusion depth on In concentration in theampoule is greater than for P. Although the dependence on P has beenpreviously demonstrated by Tien and Miller, supra, the dependence on Inis presented for the first time.

We also found that at a substrate temperature of 680 degrees C., and foran In-Cd diffusion source, the anomalous profile was produced only forCd activities in excess of about 0.15. No similar limitation wasobserved, however, for a P-Cd source. We believe that Cd activities lessthan 0.15 at this temperature effectively reduce the surfaceconcentration of acceptors below the desired upper 10¹⁸ cm⁻³ range andthus prevent the formation of two junctions.

It should be noted that the Cd activity limit is also a function ofsubstrate temperature. At higher temperatures, as the vacancyconcentration increases, a higher Cd activity than 0.15 is required. Atpresent, however, we do not understand why there is no similar limit fora P-Cd source. In any event, we expect that the anomalous profile can beattained in bulk InP using the above-described procedure with diffusiontemperatures in the range of about 630 to 850 degrees C. We havesuccessfully obtained such profiles for diffusion times in the range of1 to 4 hours at 680 degrees C.

The formation of a p⁺ -p⁻ -n⁻ junction in InP which is delineated as twodiffusion fronts by etching can be understood from FIG. 3. Curves V andVI represent the normal error function profile and the anomalous doublediffusion profile of Cd in InP, respectively. Curve VI deviates fromcurve V for depths greater than the concave section VIc of curve VI. Forthe normal diffusion profile, only the position where the diffusingspecies is equal to the substrate doping level, i.e., the p-n junctionat d₁, is revealed by etching. If the diffusion profile is that of curveVI, two possibilities arise. For substrates doped above the level of theconcave section, one front is present as discussed for curve V. However,for substrates doped (line VII) below concave section VIc, etchingreveals both the p⁻ -n⁻ junction at d₃ and the position of the concavesection in the anomalous diffusion profile, i.e., the p⁺ -p³¹ junctionat d₂.

To demonstrate the quality of the p⁺ -p⁻ -n⁻ junctions, mesa diodes werefabricated from a Cd diffused wafer with the p⁺ -p⁻ and p⁻ -n⁻ junctionsat 3.1 μm and 20.0 μm, respectively. The diffusion source comprised 3.5mg Cd and 3.4 mg In. Mesas were defined by standard photolithographytechniques and etched using 2 percent Brmethanol. Be/Au and Sn/Au wereused for the p-contact 22 and n-contact metallization 20, respectively.The I-V characteristics of these mesa diodes, 150 μm in diameter, showeda reverse breakdown voltage between 90 V and 220 V and a reverse leakagecurrent from ˜5 pA to ˜10 pA at half breakdown. The reverse I-Vcharacteristics of the diode with the highest breakdown voltageexhibited double breakdown, at 160 V and 220 V and was typical of thedevices. The two breakdowns are probably due to the variation in dopantconcentration across the p⁻ -region but are not an intrinsic property ofthe device.

For a donor carrier concentration of 4×10¹⁶ cm⁻³, the avalanchebreakdown voltage of state-of-the-art, abrupt p⁺ -n⁻ InP APDs is ˜30 V(see T. P. Lee et al, supra). The p⁺ -p⁻ -n⁻ diodes fabricated in thisstudy from n⁻ =4×10¹⁶ cm⁻³ InP crystals had breakdown voltages exceedingthat of p⁺ -n⁻ diodes by a factor of ˜3 to ˜6. The higher breakdownvoltage of the p⁺ -p⁻ -n⁻ diodes is due to the reduction of the junctionelectric field by the formation of the p⁻ -region.

It is to be understood that the above-described arrangements are merelyillustrative of the many possible specific embodiments which can bedevised to represent application of the principles of the invention.Numerous and varied other arrangements can be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention. In particular, while only InP:Cdphotodiodes have been shown by example, it is apparent that theanomalous diffusion profile in other materials using other acceptors canbe exploited in a similar fashion; e.g., Group III-V compounds such asGaAs:Zn, InP:Zn, GaP:Zn, GaAs:Mn, InGaAsP:Cd or Zn; and othersemiconductors such as Ge:Cu or Ni and CdS:Ag.

We claim:
 1. A photodiode (10) comprising:an i-type semiconductor activeregion (16) in which light (18) to be detected is absorbed to generatephotocarriers, a pair (12, 14) of opposite-conductivity typesemiconductor zones on opposite sides of said active region forcollecting said photocarriers, one (14) of said zones forming a p-i typefirst junction (15) at its interface with said active region, and theother (12) of said zones form an i-n second junction (17) at itsinterface with said active region (16), CHARACTERIZED IN THAT saidphotodiode includes a relatively low-doped semiconductor body (12),acceptors in said body from an anomalous diffusion profile (FIG. 3) ofnet impurity concentration versus depth into said body including upperand lower monotonically decreasing segments (VIa and b) connected by anupwardly concave segment (VIc), the carrier concentration (VII) of saidbody is less than the net carrier concentration corresponding to saidconcave segment, said first junction (15) comprises a p-i junction atapproximately the depth of said concave segment, said second junction(17) comprises an i-n junction at approximately a depth corresponding tothe intersection of said lower segment (VIb) and the carrierconcentration (VII) of said body, and said active region comprises acompensated i-region between said junctions.
 2. The photodiode of claim1 wherein said body comprises a Group III-V compound and said acceptoris selected from the group consisting of Mn, Zn and Cd.
 3. Thephotodiode of claim 2 wherein said body comprises n-type InP and saidacceptor comprises Cd.
 4. The photodiode of claims 1, 2, or 3 whereinsaid active region is at least partially depleted without theapplication of reverse bias to said p-n junction.
 5. The photodiode ofclaim 4 wherein said body is doped n⁻ -type to a concentration in themid-10¹⁶ cm⁻³ range.
 6. The photodiode of claim 1 wherein said activeregion is p⁻ -type, said first junction is a p⁺ -p⁻ junction, and saidsecond junction is a p⁻ -n⁻ junction.